Bacterial Pathogens (1-3) Flashcards
What is a pathogen?
A microbe roar is able to cause disease in a plant, animal or insect
What is pathogenicity?
The ability to produce disease in a host organism
→ expressed through their virulence
→ determinants of virulence: genetic, biochemical or structural features that enable it to produce a disease
What are the underlying mechanisms of bacterial pathogenicity?
Ability to invade → colonisation (adherence and multiplication), ability to overcome host defence, extracellular substances that facilitate entry
→ e.g. adhesins like fimbiae
Ability to produce toxins → exotoxins: released from bacteria e.g. Cholera toxin
→ endotoxins: cell-associated e.g. LPS, capsule
How can pathogens enter the human body?
Respiratory → inhalation
Gastro-intestinal → ingest
Urinary/genital tracts
via insect bites or accidental/surgical trauma to skin
What are some examples of human primary pathogen transmission?
Bordetella pertussis → whooping cough, requires contact with infectious material
Neisseria gonorrhoeae → sexually transmitted, requires direct person-to-person mucosal contact - man only natural host so dies in environment
What does the environment that a pathogen can colonise effect?
The environments that can be colonised by a pathogen are critical in determining its reservoirs and potential modes of transmission
What is a psychrophile, psycotroph, mesophile and thermophile?
Psychrophile → microbes that grow best at low temps (optimum 10-15C)
Psychotroph → able to grow at low temps, but prefer moderate (optimum 15-30C)
Mesophile → most bacteria (optimum 30-40C)
Thermophile → optimum 45-70C
(phile = loves)
Are bacterial growth curves symmetrical?
No → steady increase to optimum then crash after
What is the acclimation phase in cold shock?
A downshift in temp causes inhibition of most protein synthesis - has to produce new machinery (ribosomes)
→ causes a growth lag - acclimation phase
→ a group of cold shock proteins (Csp) are dramatically induced - essential for cell growth to resume at the low temp
What are E. coli cold shock proteins?
Involved in ensuring ribosome function and DNA/RNA synthesis
Class I → >10-fold induction
→ e.g. CspA family: CspA RNA chaperones, CspB/CspC RNA&DNA chaperone, RNP ribonuclease
Class II → <10-fold induction
→ e.g. RecA: recombination factor, H-NS: nucleoid-assocaited DNA-binding protein
→ conditioning ribosome for cold temps.
What is Listeria?
Non-spore forming Gram +ve bacilli
→ causes listeriosis
→ serious for pregnant women and immunocompromised people
→ widespread in environment, vegetation, water4, soil, wild/domestic animals, fish
What is unique about Listeria bacterial growth?
Capable of growth over a wide range of temperatures (4-40C)
→ e.g. can survive in fridge
→ can be spread through contaminated food: meats, unpasteurised milk, vegetables (coleslaw) - foods that require no further heat treatment
What virulence factors of Listeria is involved in the invasion of mammalian cells?
Internalin A and B
What virulence factors of Listeria are required for its escape from membrane vacuole?
LLO (listeriolysin) → pore-forming cytotoxin punches holes in membrane - surrounded by vacuole upon entry so must degrade
PI-PLC (PlcA) → an enzyme that removes charged head groups from phospholipids
double membrane vacuole:
PC-PLC (PlcB) → a phospholipase that cleaves the head group from many kinds of lipids
What virulence factors of Listeria are required for movement through the cytoplasm (cell-cell spread)?
ActA → stimulates host cell actin polymerisation (in wrong place ie. surface of bacterium), actin tail forms and propels bacterium through cytoplasm
→ causes projections from the host cell enter adjacent cells
How are Listeria virulence factors regulated?
PrfA → positive regulator of virulence genes
→ may repose to temp
→ DNA binding protein
How is Listeria controlled in food?
Can grow and multiply at normal refrigeration temps, freezing, and relatively high cooking temps - also can grow in other food preservatives
→ controlled by: heating at 70C for 2 mins, avoiding unpasteurised milk, avoid coleslaw and deli meats, cook meat thoroughly and reheat after refrigeration
What is Legionella pneumophila?
Motile, aerobic Gram -ve rod
→ cause Legionnaire’s disease - a form of bacterial pneumonia
→ normal environment: biofilm (in air con) or inside protozoa - humans accidental host
How does pH affect growth rate?
Growth rate curve symmetrical around the optimum pH
What are the terms used in relation to pH and growth?
Acidophiles → grow at an optimum below neutrality (>7pH)
Neutrophiles → grow best at neutral pH
Alkaliphiles → grow best under alkali conditions
What is an obligate acidophile?
Require a low pH for growth
→ their membranes dissolve and cells lyse at neutrality
→ several genera of archaea
What are obligate alkaliphiles?
Can grow around pH 10
→ Na+ gradient (instead of pmf = proton motive force = pH gradient and electrochemical gradient) supplies the energy
What does the intracellular pH for most organisms need to be?
Intracellular pH mostly needs to stay around neutrality
→ to prevent destruction of acid or alkali labile macromolecules
anomalies: extreme acidophiles and alkaliphiles
→ their internal pH can very by several units from neutrality
What are the problems posed by acidic conditions?
3 major aspects of cell function cane affected by acidic conditions
1. the capacity for nutrient acquisition and energy generation
2. cytoplasmic pH homeostasis
3. protection of proteins and DNA → critical for cell survival
How can proteins and DNA be protected in acidic conditions?
Chaperone proteins and alkalisation of the periplasm
(periplasm in Gram -ve cell walls, between outer and cytoplasmic membrane)
What is Helicobacter pylori?
Gram -ve curved rod, highly motile 4-7 polar flagella
Cause gastric and duodenal ulcers (associated with development of gastric cancer)
Reservoir: lining of the stomach
Main virulence factors: flagella, urease, adhesions,
How do we know Helicobacter pylori causes ulcers?
Known to be associated but there was no animal model so couldn’t follow usual method to confirm Koch’s postulates
→ Barry Marshall drank culture of H. pylori - 2 weeks later had inflamed stomach
How does H. pylori survive in the acidic pH of the stomach?
Organism doesn’t colonies the lumen but the mucin layer that covers the stomach
→ mucus resists diffusion of protons from stomach acid due to -ve sulphated polysaccharides
However must reach the mucin layer so needs:
→ motility
→ short term pH protection using enzyme urease
What does the enzyme urease do?
Urease takes up urea (waste product) and hydrolyses it producing ammonia and CO2
→ ammonia neutralises stomach acid in its immediate vicinity
→ urease is intraceullular so exports ammonia to periplasm where pH can rise ~ pH 6
How does Helicobacter pylori give rise to ulcers?
Uses urease to protect it from stomach acid during transit
→ colonises mucin layer
Products provoke inflammatory response that ultimately damage the mucosa
BabA: adhesin that recognises Lewis b antigen which binds sulphated mucin sugars on epithelial cells
NAP: neutrophil activation protein - activates neutrophils, leads to inflammation
VacA: vacuolating cytotoxin - produces large vacuoles in mammalian cells
What is Salmonella typhimurium?
Characteristics: Gram-negative rod, motile
Disease: gastroenteritis, diarrhoea
Reservoir: human carriers, livestock animals, reptiles
Transmission: contaminated food
Main virulence factors: acid tolerance response, adhesins, invasion of mucosal cells, type III secretion system
Prevention: proper food handling
Treatment: antibiotics
What is the Salmonella typhimurium acid tolerance response?
Allows S. typhimurium to survive in the acid environment of the stomach
Rapid exposure to low pH → rapid die-off
More gradual exposure to low pH (i.e. adapted to pH6 first) → cells can survive down to pH 3
What cells regulate acid shock proteins?
Fur = the regulator of iron acquisition genes
→ Fur senses pH and iron separately
→ iron is an essential micro-nutrient
How can pH tolerance in Gram + bacteria affect human health?
mutans Streptococci → causes dental caries - organisms produce acid and can survive at low pH
Listeria → causes listeriosis - drop in pH activates haemolysin permitting its escape from the phagosome, capable of surviving acid stress in food like cottage cheese, yoghurts and orange juice
Rhodococcus equi → causes broncopneumonia in horses - encounters low pH in alveolar macrophages - acid resistant
What are the mechanisms of acid resistance in Gram positive bacteria?
Proton pumps → F1F2-ATP ases, GAD
Protein/DNA repair → some chaperones
Regulators → e.g. several 2 component systems (sensor + response) in Listeria
Altered metabolism → e.g. a quorum sensing system & biofilm - acid tolerance in S. mutans
Envelope alterations → e.g. S. mutans increased levels of mono-saturated and longer chain fatty acids at pH5 than pH7
Production of alkali → e.g. urease production (increased pH)
What are obligate aerobes and anaerobes?
Aerobes: require O2 for growth
→ they use O2 as a final electron acceptor in aerobic respiration
Anaerobes: don’t need O2 as a nutrients (O2 is toxic - kills or inhibits growth)
→ may live by fermentation, aerobic respiration, bacterial photosynthesis or methanogensis
What are facultative anaerobes?
Organisms that can switch between aerobic and anaerobic metabolism
→ no O2 - fermentation, anaerobic respiration
→ O2 - aerobic respiration
What are aerotolerant anaerobes?
Bacteria with an exclusively anaerobic (fermentative) type of metabolism but are insensitive to presence of O2
→ live by fermentation alone whether of not O2 is present
How do flavoproteins react with O2?
Oxidation of flavoproteins results in the formation of H2O2 (peroxide) and small quantities of O2- superoxide (an even more toxic free radical)
→ ROS can damage cells - will kill the cell if nothing is done about them
How is the potential for lethal accumulation of superoxide prevented in aerobes?
The enzyme superoxide dismutase
→ all organisms which can live in the presence of O2 contain superoxide dismutase
How is peroxide decomposed?
By the enzyme catalase
→ certain aerotolerant bacteria lack catalase so they use peroxidase enzymes (derive e- from NADH2 producing water)
Why do obligate anaerobes undergo lethal oxidations by various oxygen radicals when exposed to O2?
They lack superoxide dismutase and catalase
→ can’t protect against ROS
What is the action of superoxide dismutase, catalase and peroxidase?
Detoxify oxygen radicals
Superoxide dismutase: [O₂-] + [O₂-] (+2H+) → O₂ + H₂O₂
Peroxidase: H₂O₂ + O₂ (+ NADH + H+ → NAD+) → 2H₂O
Catalase: 2H₂O₂ → 2H₂O + O₂
How does Clostridium spp. obtain ATP?
Substrate-level phosphorylation
→ most lack respiratory chain cytochromes, catalase, peroxidase and superoxide dismutase
What is Clostridium botulinum?
Commonly found in soil samples and aquatic sediments
→ causative agent of botulinum food poisoning
→ releases exotoxin bolutlinum - extremely potent + can form spores
→ poorly canned foods create anaerobic environment - unskilled spores germinate and produce toxin
What is botulism?
Result of ingesting bacterially produced neurotoxins
→ the toxins block the release of neurotransmitter acetylcholine
→ resulting in double vision, slurred speech, decreased saliva, general weakness
→ paralysis with accompanying respiratory failure can be fatal
What are the types of exotoxin produced by Clostridium botulinum?
7 - A, B, C1, D, E, F, G
→ A, B, E, and F are the most toxic to humans (often released in inactive form, proteolytic cleavage activates them)
→ A is the most potent exotoxin known
What is the mode of action of botulinum toxin?
Botulinum toxin has heavy chain (HC) and light chain (LC)
→ HC binds toxin to the presynaptic receptor - toxin enters the cell
→ disulphide bond cleaved - liberates the LC into cytoplasm and endosomal compartment
→ cleaves the SNARE proteins (involved in vesicle docking)
→ prevents fusion of acetylcholine vesicles at the cell membrane
Prevents the release of Act → no stimulus → can’t contract → flaccid paralysis
What is Clostridium tetani?
Causative agent of tetanus (lockjaw)
→ result of trauma/puncture wound leading to tissue contamination
→ caused by release of single antigenic type endotoxin by C. tetani - circulates in blood and adheres to neuronal receptors
→ fixes to gangliosides - blocking release of the neurotransmitters glycine and y-amino butyric acid (GABA)
→ muscles permanently contracted
How does the tetanus toxin work?
Tetanus toxin made up of a heavy chain (HC) required for cell entry and a light chain (LC) which causes disease
→ C-terminal domain of the HC binds to gangliosides, N-terminal domain allows LC to cross into cell cytoplasm
→ LC interrupts the release of neurotransmitters
→ cleaves SNARE proteins - GABA and glycine containing vesicles can’t dock thus no neurotransmitter released
Glycine usually induces muscle relaxation - constant release of Act - muscles permanently contracted
How are hosts affected by Clostridium tetani?
Toxin causes tetanus
→ host usually dies from respiratory paralysis
heroin addicts are particularly susceptible
What is Clostridium difficile?
Gram positive, obligately anaerobic, spore forming
Antibiotic use reduces normal microbiota → C. diff overgrows produces toxins A and B
→ A and B - large exotoxins that modify host cell membrane G proteins
Toxins cause: diarrhoea and lesions on colon surface
→ coalesce forming tissue damage - pseudomembranous colitis
→ can be rapidly fatal
